The disclosure relates generally to thermally conditioned architectural glass, and specifically relates to thermally strengthened architectural glass and to related methods and systems for the thermal strengthening of architectural glass, particularly for thin architectural glass sheets.
In thermal (or “physical”) strengthening of architectural glass sheets, an architectural glass sheet is heated to an elevated temperature above the glass transition temperature of the glass and then the surfaces of the sheet are rapidly cooled (“quenched”) while the inner regions of the sheet cool at a slower rate. The inner regions cool more slowly because they are insulated by the thickness and the fairly low thermal conductivity of the architectural glass. The differential cooling produces a residual compressive stress in the architectural glass surface regions, balanced by a residual tensile stress in the central regions of the architectural glass.
Thermal strengthening of glass is distinguished from chemical strengthening of glass, in which surface compressive stresses are generated by changing the chemical composition of the glass in regions near the surface by a process such as ion diffusion. In some ion diffusion based processes, exterior portions of glass may be strengthened by exchanging larger ions for smaller ions near the glass surface to impart a compressive stress (also called negative tensile stress) on or near the surface. The compressive stress is believed to limit crack initiation and/or propagation.
Thermal strengthening of glass also is distinguished from glass strengthened by processes in which exterior portions of the glass are strengthened or arranged by combining two types of glass. In such processes, layers of glass compositions that have differing coefficients of thermal expansion are combined or laminated together while hot. For example, by sandwiching molten glass with a higher coefficient of thermal expansion (CTE) between layers of molten glass with a lower CTE, positive tension in the interior glass compresses the outer layers when the glasses cool, again forming compressive stress on the surface to balance the positive tensile stress. This surface compressive stress provides strengthening.
Thermally strengthened architectural glass has advantages relative to unstrengthened architectural glass. The surface compression of the strengthened architectural glass provides greater resistance to fracture than unstrengthened architectural glass. The increase in strength generally is proportional to the amount of surface compression stress. If a sheet possesses a sufficient level of thermal strengthening, relative to its thickness, then if the sheet is broken, generally it will divide into small fragments rather than into large or elongated fragments with sharp edges. Glass that breaks into sufficiently small fragments, or “dices,” as defined by various established standards, may be known as safety glass, or “fully tempered” glass, or sometimes simply “tempered” glass.
Because the degree of strengthening depends on the temperature difference between the surface and center of the glass sheet during quenching, thinner glasses require higher cooling rates to achieve a given stress. Also, thinner glass generally requires higher values of surface compressive stress and central tension stress to achieve dicing into small particles upon breaking. Accordingly, achieving desirable levels of tempering in glass with thicknesses of around 3 mm or less has been exceedingly challenging, if not impossible.
Aspects of the present disclosure also relate generally to an architectural glass or glass-ceramic that has a stress profile for strengthening exterior portions thereof. Architectural glass and glass-ceramic articles, such as sheets of architectural glass, may be used for a broad range of applications. Examples of such applications include use in architectural windows, single and multi-pane windows, indoor and outdoor windows, vacuum insulated glass windows, and safety glass windows in buildings, homes, hotels, offices, and other similar structures.